1. Field of the Invention
The invention relates to a method for driving semiconductor switches a series circuit and to a circuit arrangement for carrying out this method.
2. Discussion of the Background
It is generally known that semiconductor switches--such as power transistors, MOSFET transistors, IGBTs (insulated gate bipolar transistor) and GTOs (gate turn-off thyristor)--have a limited maximum reverse voltage. In the event that higher voltages are to be switched, it is necessary to connect the semiconductor switches in series. In this arrangement, the switched DC voltage exceeds the maximum permitted voltage of each of the semiconductor switches connected in series. In consequence, a protective device must ensure that the maximum permitted voltage on each semiconductor switch is not exceeded, irrespective of the operating condition. This is the case if the DC voltage is distributed uniformly across the individually opened semiconductor switches. A uniform or symmetrical voltage distribution over the switched-off semiconductor switches is normally achieved in steady operation by means of balancing resistors.
In dynamic operation, that is to say during a switch-on or switch-off process, the symmetrical voltage distribution across the semiconductor switches is only ensured if the switching behavior of the individual semiconductor switches is identical. Nevertheless, it is known that the switching behavior or the switch-on and switch-off delay, respectively, of the individual semiconductor switches is different. The dissimilar switch-on and switch-off times, moreover, vary with time and as a function of the temperature. This leads to an equally rapid switch-on or switch-off of the individual semiconductor switches not being guaranteed at all operating points. Thus, the more rapid semiconductor switch accepts a higher voltage during the switching off, because the slower semiconductor switch is still conducting. In consequence, the maximum reverse voltage U.sub.cmax of the most rapid semiconductor switch can be exceeded, so that it is destroyed. The same condition also occurs in the case of unsymmetrical switching on of the semiconductor switches. In this case, the slowest semiconductor switch accepts the full voltage, since all the remaining semiconductor switches are already switched on and accept a lower voltage.
The publication JP-A-55-033313 discloses a circuit arrangement for driving semiconductor switches connected in series, in which the balancing of the switching behavior is guaranteed by means of an individual displacement of the switching commands of the individual semiconductor switches. In this arrangement, the most rapid semiconductor switch receives its control pulse with a maximum delay and the slowest semiconductor switch receives the control pulse immediately, so that all the semiconductor switches finally switch simultaneously.
The technical solution described in publication EP-A-0202962 further proposes to determine the individual displacement of the switching commands of the individual semiconductor switches by means of a measuring technique, in that, for example, the actual transistor switching edges are measured and the necessary switching delay is determined therefrom. This solution is, however, not able to be applied in practice in the case of IGBTs because of the extremely short delay times which can be detected only inaccurately with a measuring technique.
In addition, it can be gathered from the teaching of publication EP-B1-0288422 to measure the steady voltages of the semiconductor switch and to control them to a prescribed desired value by displacing the switching edges. In this case, the procedure is that if the semiconductor voltage in the last switch-off was too large, the next switch-off edge is delayed by a controller. If the voltage of the preceding switch-off was too low, the switch-off edge is displaced to an earlier instant.
One problem of the last two solutions consists in the fact that the first switchings off are carried out uncompensated, because the individual delay time of the respective semiconductor switch must first be built up. The known switching arrangements and, respectively, the methods used in the latter for driving the semiconductor switches can consequently only be used under the condition that the first switchings off are carried out at a reduced DC voltage, so that the corresponding control device can align with the switching behavior, before the full operating voltage or the full operating current, respectively, is built up. Apart from this restriction, a further problem exists in connection with the protection of the semiconductor switches in the event of failure or disturbance of the control equipment for the voltage. In this respect, there is no usable solution to date.
Also known from the prior art is a method for the protection of serially connected semiconductor switches, a voltage limiting device being assigned to each semiconductor switch for carrying out the method. In the case of high voltages on the semiconductor switch, which exceed a specific value and thereby activate the voltage limiting device, a large current flows through the voltage limiting device at a high voltage. In so doing, pulse-like electrical losses occur in the voltage limiting device, which considerably reduce the lifetime of the voltage limiting device in the event of repeated switching processes.